I wrote a merge sort. When I changed the 35 lines of code to malloc(NULL), I found that the program could still sort the results without reporting an error. Why is this happening?
There is a warning at malloc(NULL) but no error.
int sort(int*a,int n){
int *p=(int *)malloc(NULL);
g_sort(a,0,n-1,p);
return 1;
}
The code can be run to sort the correct result:
#include<bits/stdc++.h>
using namespace std;
void m_qsort(int *a,int left,int mid,int right,int*temp){
int m=mid;int r=right;
int k=0,j=mid+1;;
int l=left;
while(l<=m && j<=r){
if(a[l]<=a[j])
temp[k++]=a[l++];
else
temp[k++]=a[j++];
}
while(l<=mid){
temp[k++]=a[l++];
}
while(j<=r){
temp[k++]=a[j++];
}
for(int i=0;i<k;i++){
a[left+i]=temp[i];
}
}
void g_sort(int *a,int left,int right,int*temp){
if(left<right){
int mid=(left+right)>>1;
g_sort(a,left,mid,temp);
g_sort(a,mid+1,right,temp);
m_qsort(a,left,mid,right,temp);
}
}
int sort(int*a,int n){
int *p=(int *)malloc(NULL);
g_sort(a,0,n-1,p);
return 1;
}
int main()
{
int a[22]={8,4,7,5,6,2,1,7,10,25,0,1,75,52,14,56,33,24,852,20,26,41};
sort(a,22);
for(int i=0;i<22;i++){
cout<<a[i]<<' '<<endl;
}
}
Result:
$g++ -o main *.cpp
main.cpp: In function ‘int sort(int*, int)’:
main.cpp:46:27: warning: passing NULL to non-pointer argument 1 of ‘void* malloc(size_t)’ [-Wconversion-null]
int *p=(int *)malloc(NULL);
^
$main
0
1
1
2
4
5
6
7
7
8
10
14
20
24
25
26
33
41
52
56
75
852
